1. Field of the Invention
The present invention generally relates to test fixtures for circuit boards and, more particularly, to an adjustable tooling pin for positioning electronic circuit boards in a vacuum test fixture.
2. Description of the Prior Art
Circuit board designers are constantly faced with the challenge of packaging more and more electronics into a smaller amount of space. Often times, during the design process, circuit test pads must be relocated to other places on the board to make room for other functional components. Test pads are necessary on circuit boards to allow interconnection to test equipment via a test fixture. A test fixture is typically an automated device which comprises a plurality of fixed upwardly projecting spring-loaded test probes. This type of fixture is commonly referred to as a "bed-of-nails" test fixture. Each of the test probes in the bed correspond to a test pad on the circuit board such that when the board is pulled down on top of the probes, each probe is brought into electrical contact with its corresponding test pad.
FIG. 1 shows an example of a bed-of-nails test fixture for testing circuit boards (cards) generally referred to by the reference numeral 10. The test fixture 10 comprises a probe plate 12 having a plurality of upwardly projecting spring-loaded test probes 14 which resemble a bed-of-nails. Each of the test probes 14 correspond to a test pad 16 on the card 18 under test. The card 18 sits on blocks 20 above a rigid top plate 22. A tooling pin 24, fixed to top plate 22, fits into an alignment or tooling hole in the card 18 to provide alignment for the test probes 14 and the test pad 16. The guide plate must be made of a non-conductive material so as not to interfere with the electrical signals from the tester. When a vacuum chamber 28, sealed by a vacuum mat 30 and vacuum seal 32, is evacuated of air, atmospheric pressure pushes down on the vacuum mat 30 and top plate 22 compressing springs 26 causing the top plate 22 to move in a downward direction along guide pin 34. A backer plate 36, connected to the guide plate 22, pushes down on backer post 38 which, in turn, pushes down on the circuit board 18 under test. Guide apertures 40 allow probe tips 42 to slide therethrough and make electrical contact with the test pad 16.
A drawback to the test fixture shown in FIG. 1 is the non-adjustable design of the tooling pins 24. This is a problem since card assembly processes constantly change the tooling pin locations. For example, heat from wave solder processes or heat created at card burn-in can change the tooling hole location. The tooling pins 24 must accurately locate the card under test 18 to spring loaded test probes 14 in the test fixture to make sure that each test probe makes good electrical contact to its corresponding test pad 16. This leads to particular alignment problems for test pads located on 25 mil (0.063 mm), 40 mil (0.101 mm) and 50 mil (0.127) drill centers since the tooling pins 24 are not adjustable.
There are two methods used for aligning the card 18 under test with the bed of test probes 14. The first method comprises physically changing the location of the tooling pin 24 by plugging the original tooling hole in the top plate 22 and redrilling at a new location. This effort may need to be repeated for each batch of cards where the test pads have changed to again realign the card 18 to the test probes 14. This particular method of alignment is time consuming and inefficient and can only be preformed a finite number of times before the top plate 22 has too many plugged holes to be useful.
The second method of realigning the card 18 under test with the bed-of-nails test probes 14 involves the use of a sophisticated optical alignment system with micrometer adjustments. However, this method is prohibitively expensive.
As technology drives the sizes of components and test pads down, it becomes increasingly important to have an accurately aligned tooling pin as well as a method for making minute and frequent adjustments at the tester itself while the product is undergoing test.